This invention relates to an apparatus for determining the minority carrier diffusion length in semiconductors using the constant-magnitude surface photovoltage (SPV) method.
Apparatus and methods for determining the minority carrier diffusion length (L) using the SPV method are well known. In brief, the principle of the diffusion length (L) determination requires the illumination of a specimen surface with monochromatic radiation of energy slightly greater than the bandgap of the semiconductor. Electron-hole pairs are produced and diffuse to the illuminated (front) surface where they are separated by the electric field of the depletion region (i.e., the surface-space-charge region) to produce a surface photovoltage (SPV). A portion of the SPV signal is coupled to an amplifier for amplification and measurement. The photon intensity (photons per sq. cm. per second) is adjusted to produce the same magnitude of SPV at various wavelengths of illumination. The photon intensity rcquired to produce this constant magnitude SPV signal is conveniently plotted on the ordinate against the reciprocal of the absorption coefficient on the abscissa for each wavelength. The resultant plot is linear and is extrapolated to the zero intensity intercept on the negative abscissa. This intercept value is the effective diffusion length (L). For a more detailed description of the theory and background for this method, see an article "A Method for the Measurement of Short Minority Carrier Diffusion Lengths in Semiconductors," by A. M. Goodman in the Journal of Applied Physics, Vol. 32, No. 12, pp. 2550-2552, December 1961. The American Society for Testing and Materials has adopted a standard using this method which is published as ASTM F 291-78. The ASTM standard, when implemented according to the block diagram of FIG. 1 of ASTM F 291-78, is provided particularly for testing the diffusion length (L) for minority carriers in silicon but the method in general may be used for other semiconductor materials.
See U.S. patent application, entitled "METHOD AND APPARATUS FOR DETERMINING MINORITY CARRIER DIFFUSION LENGTH IN SEMICONDUCTORS", Ser. No. 153,920, now U.S. Pat. No. 4,333,051, filed on May 28, 1980 by A. M. Goodman for a description of an apparatus using this principle in which a servo system maintains a constant predetermined value of the SPV thereby allowing the measurements to be carried out in a relatively short time. This minimizes the effects of drift caused by laterally diffusing minority carriers during a test. This Goodman application describes a capacitance-pickup electrode which is placed to make physical contact with the surface of a semiconductor material to sense SPV test signals. The Goodman method however is not suitable for measuring the significantly small diffusion length of amorphous silicon which is known to exhibit very slow transients in SPV generated by chopped or modulated light.
See U.S. patent application, entitled "METHOD AND APPARATUS FOR DETERMINING MINORITY CARRIER DIFFUSION LENGTH IN SEMICONDUCTORS," Ser. No. 228,575, now U.S. Pat. No. 4,393,051, filed on Jan. 26, 1981 by B. Goldstein et al. for a description of an apparatus by which the SPV is measured by a Kelvin method-type probe.
In my copending U.S. patent application, entitled "METHOD AND APPARATUS FOR DETERMINING MINORITY CARRIER DIFFUSION LENGTH IN SEMICONDUCTORS," Ser. No. 280,918, now U.S. Pat. No. 4,443,288, filed July 6, 1981, I describe a means for determining the minority carrier diffusion length using a redox solution supported in an open-ended cell placed on the surface of a semiconductor material. That apparatus is difficult to move over the surface since the redox liquid must be removed and replaced for each measurement at a different location. Moreover, since the cell carrying the redox solution is relatively deep, correction is needed for the effect of the colored redox solution on the measurements.
There is a need for a means to determine the SPV in semiconductors by a direct connection to the surface of material using steady or very low frequency varying light using a redox solution that is easily moved over the surface and requires no correction for the color effect of the solution.